1. Field of the Invention
The present invention relates to a sheet thickness detection device for detecting a thickness of a sheet being conveyed, and an image forming apparatus mounted with the sheet thickness detection device.
2. Description of the Related Art
An image forming apparatus such as a printer and a multi-function peripheral is supplied with, e.g., normal sheets or coated sheets from a sheet feeding apparatus disposed inside or outside the image forming apparatus, forms images on respective ones of the supplied sheets, and then outputs these sheets as printed products. In some image forming apparatus, a sheet property (e.g., sheet thickness) is detected to stably form high-quality images.
The sheet feeding apparatus incorporates a storage container in which sheets are stored, and is configured to separate and feed sheets one by one from the storage container to the image forming apparatus.
However, two or more sheets are sometimes conveyed together due to separation failure (hereinafter, referred to as the multiple feed). In that case, there is a fear in electrographic image formation that an image failure or a fixing failure occurs when a toner image is transferred and fixed to a sheet and a conveyance failure occurs due to increase in conveyance resistance in a conveyance path of the sheet feeding apparatus or of the image forming apparatus.
To prevent occurrences of these failures, a multiple feed detection system has been proposed that detects multiple sheet feed by using, e.g., an optical sensor. This multiple feed detection system detects a thickness of a sheet being conveyed to thereby discriminate between single sheet conveyance and multiple sheet conveyance.
In electrographic image formation, the thermal capacity of a sheet (an amount of heat removed by the sheet) varies depending on the thickness of the sheet, so that an amount of heat applied to a toner image on the sheet becomes unstable, resulting in a problem of fixing property. To ensure the fixing property, it is necessary to grasp the sheet thickness and to apply the toner image on the sheet with an amount of heat determined by taking account of the sheet thickness.
With a conventional image forming apparatus, the amount of heat applied to the toner image is varied according to sheet thickness information input by a user to the image forming apparatus, whereby a stable fixing property is ensured.
As described above, the electrographic image forming apparatus and the sheet feeding apparatus require a sheet thickness detection apparatus capable of detecting the sheet thickness in order to prevent multiple sheet feed and image failure and to ensure a stable fixing property.
To meet the above demand, a paper sheet thickness detection mechanism disclosed in, e.g., Japanese Patent Publication No. 2872022 has a conveyance path on which there are provided a process start trigger sensor for detecting a paper sheet and for starting a thickness detection process and a detection roller disposed apart from a fixed reference roller by a distance corresponding to paper sheet thickness. The detection roller is displaced according to the thickness of a sheet that runs to the reference roller. If a paper sheet which is excessively large in thickness is fed or if multiple paper sheets are fed simultaneously, the detection roller is largely displaced.
When the detection roller is displaced according to paper sheet thickness, the amount of displacement of the detection roller is enlarged by a gear ratio and an arm ratio, and the enlarged displacement is conveyed to a scale formed with slits which are vertically arranged at equal intervals. An encoder counts light and dark slits and outputs rectangular pulses which are the same in number as the number of counts. By measuring the output of the encoder, it is possible to detect a sheet thickness or the number of multiple sheets fed simultaneously. However, the conventional sheet thickness detection device entails the following problems.
In recent electrophotographic image forming apparatuses such as copying machines and multifunction peripherals, especially in printers, there are a variety of types of sheets being conveyed.
First, the sheet thickness varies in an extremely wide range from 38 g/m2 to 350 g/m2 in terms of sheet basis weight. For a sheet which is small in basis weight and thickness and has small rigidity, there is a fear that the sheet is bent at its end portion and wrinkled by a conveyance roller during being conveyed, which requires that the arrangement for conveyance be configured taking into account this point.
Secondly, the sheet size in conveyance direction varies in a range from B5 (182 mm) to 18 inch (about 460 mm). In a high-speed image forming apparatus where sheet conveyance is performed at a high speed (e.g., 1300 mm/sec), a time available for sheet thickness detection is extremely short unless the sheet conveyance is stopped or decelerated. Accordingly, it is necessary that the apparatus be configured taking into account the response in sheet thickness detection.
However, the paper sheet detection apparatus heretofore proposed is configured to apply a constant roller nip pressure to a sheet between the detection roller and the reference roller. Therefore, in an arrangement configured to apply a nip pressure that enables conveyance of sheets having a relatively large basis weight (e.g., about 150 g/m2), the following problems are caused in conveying sheets having a small basis weight (e.g., 52 g/m2). Since the rigidity of a sheet which is small in basis weight is insufficient to withstand the nip pressure between the detection roller and the reference roller, the sheet is curled at its end or the entire sheet is wrinkled, which results in conveyance failure or results in deteriorated product quality.
On the other hand, in an arrangement configured to apply a nip pressure that enables conveyance of sheets having a relatively small basis weight (e.g., about 52 g/m2), the following problems are caused in conveying sheets having a large basis weight (e.g., 300 g/m2). Since the nip pressure between the detection roller and the reference roller becomes too small for sheets with a large basis weight and with a strong rigidity, the detection roller vibrates due to strong sheet rigidity, so that a long time is required to obtain a stabilized sensor output, which results in a remarkable reduction in response. As a result, there is a fear that sheet thickness is erroneously detected.